'Libration' definitions:

Definition of 'libration'

noun

(astronomy) a real or apparent slow oscillation of a moon or satellite; "the libration of the moon"

From: GCIDE

Libration \Li*bra"tion\ (l[-i]*br[=a]"sh[u^]n), n. [L. libratio: cf. F. libration.]
1. The act or state of librating. --Jer. Taylor. [1913 Webster]
2. (Astron.) A real or apparent libratory motion, like that of a balance before coming to rest. [1913 Webster]
Libration of the moon, any one of those small periodical changes in the position of the moon's surface relatively to the earth, in consequence of which narrow portions at opposite limbs become visible or invisible alternately. It receives different names according to the manner in which it takes place; as: (a) Libration in longitude, that which, depending on the place of the moon in its elliptic orbit, causes small portions near the eastern and western borders alternately to appear and disappear each month. (b) Libration in latitude, that which depends on the varying position of the moon's axis in respect to the spectator, causing the alternate appearance and disappearance of either pole. (c) Diurnal or parallactic libration, that which brings into view on the upper limb, at rising and setting, some parts not in the average visible hemisphere. [1913 Webster]